It is problems in Excel

Chapter Eight
Fort Langley Pizza offers no-charge delivery for orders over $20. Customers often tip the delivery drivers though. The following is a sample of 12 tips received by drivers last night.
$2.25
$1.50
$2.50
$0.00
$2.25
$2.00
$2.00
$1.50
$2.00
$2.00
$1.50
$3.00
What is the best point estimate for these tips?
If the manager wants to develop an 80% confidence interval, what distribution should be used to obtain the critical value? (Hint: think about whether you know the population standard deviation or not.)
Construct and interpret the 80% confidence interval.

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Chapter Eight
Fort Langley Pizza offers no-charge delivery for orders over $20. Customers often tip the delivery drivers though. The following is a sample of 12 tips received by drivers last night.
$2.25
$1.50
$2.50
$0.00
$2.25
$2.00
$2.00
$1.50
$2.00
$2.00
$1.50
$3.00
What is the best point estimate for these tips?
If the manager wants to develop an 80% confidence interval, what distribution should be used to obtain the critical value? (Hint: think about whether you know the population standard deviation or not.)
Construct and interpret the 80% confidence interval.
If the manager were to change her mind and ask for a 90% confidence interval instead, what is the impact on the interval width? Discuss why this occurs.
Construct and interpret a 50% confidence interval and a 99% confidence interval.
Ans:
a. Mean is the best point estimate in this case. The mean is $1.88
b and c. The sample s.d. is .7347. The distribution used , in this case, is t-distribution with 11 d.f. The 80% cut off is 1.363.
Hence the 80% CI is [ 1.88 ± 1.363*7347/v12] = [ 1.591, 2.169]
[t distribution is used instead of normal as we do not know the actual s.d.]
d. If the manager were to change her mind and ask for a 90% confidence interval instead, the interval width would increase. This is because the cut-off value will increase with increase in probability.
e. The sample s.d. is .7347. The distribution used , in this case, is t-distribution with 11 d.f. The 50% cut off is .6974
Hence the 50% CI is [ 1.88 ± .6974*7347/v12] = [ 1.732, 2.028]
The 99% cut off is 3.106
Hence the 99% CI is [ 1.88 ± 3.106*7347/v12] = [ 1.221, 2.539]
Interpretation of 90% confidence interval: various interpretations of a confidence interval can be given. The confidence interval can be expressed in terms of samples (or repeated samples): "Were this procedure to be repeated on multiple samples, the calculated confidence interval (which would differ for each sample) would encompass the...

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